Measuring the tip-sample separation in dynamic force microscopy

We have studied the dependence of the tip-sample separation with the cantilever oscillation amplitude during imaging and manipulation of Au clusters deposited on a functionalized Si substrate. By simultaneously recording the cantilever deflection and oscillation amplitude, as the Atomic Force Microscope (AFM) tip is scanned over a feature with the feedback off, one can elucidate whether the tip was tapping on the substrate or what was the minimum distance between the tip and the substrate. The Si cantilevers that we used presented two different resonance peaks and the experiments show that the selection of tile resonance peak has a large influence on the tip- sample separation. When the cantilever is driven close to the high frequency (HF) peak (≃ 305 kHz), the tip oscillates far (> 4nm) from the surface and without contacting it. For driving frequencies close to the low frequency (LF) peak (≃ 190 kHz), the tip oscillates close to the surface and taps on the substrate on each oscillation cycle. We find that the tip-sample separation varies linearly with the oscillation amplitude in both cases. In addition, the methods used to calibrate the deflection signal, i.e., convert from volts to nanometers, are discussed in detail. Knowledge of the actual distance between tip and sample can play an important role in improving nanomanipulation using Scanning Probe Microscopy (SPM). For example, the tip can be lowered and be positioned at the optimal height for manipulation without risk of crashing into the sample.